This invention relates to an improved multiple deposition chamber apparatus for manufacturing photovoltaic devices wherein successive amorphous-silicon alloy semiconductor layers are continuously deposited on a substrate moving through the deposition chambers.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloys, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n and other type devices which are, in operation in photovoltaic and other applications, substantially equivalent to their crystalline counterparts.
It is now possible to prepare amorphous silicon alloys by glow discharge techniques that have (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) provide high quality electronic properties. This technique is fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent To Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980 and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, fluorine introduced into the amorphous silicon semiconductor operates to substantially reduce the density of the localized defect states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by one or more smaller band gap materials to absorb the light passed through the preceeding cell or layer.
It is of great commercial importance to be able to mass produce photovoltaic devices. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can now be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in the following U.S. patents and pending patent applications: U.S. Pat. No. 4,400,409 issued 8/28/1983 for A Method of Making P-Doped Silicon Films And Devices Made Therefrom; Ser. No. 244,386, filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Material; U.S. Pat. No. 4,410,558, issued 10/18/1983 for Continuous Amorphous Solar Cell Production System; Ser. No. 306,146, filed Sept. 28, 1981 for Multiple Chamber Deposition And Isolation System And Method; and Ser. No. 359,825, filed Mar. 19, 1982 for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these applications, a substrate formed from stainless steel, for example, may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific material.
In making a solar cell of p-i-n type configuration, the first chamber is dedicated for depositing a p-type amorphous silicon alloy, the second chamber is dedicated for depositing an intrinsic amorphous silicon alloy, and the third chamber is dedicated for depositing an n-type amorphous silicon alloy. Since each deposited alloy, and especially the intrinsic alloy must be of high purity, the deposition environment in the intrinsic deposition chamber is isolated from the doping constituents within the other chambers to prevent the diffusion of doping constituents into the intrinsic chamber. In the previously mentioned patent applications, wherein the systems are primarily concerned with the production of photovoltaic cells, isolation between the chambers is accomplished by gas gates through which unidirectional gas flow is established and through which an inert gas may be "swept" about the web of substrate material.
In the previously mentioned patent applications, deposition of the amorphous silicon alloy materials onto the large area continuous substrate is accomplished by glow discharge decomposition of the process gases. Among these processes, radio frequency energy glow discharge processes have been commonly suggested as being suitable for the continuous production of photovoltaic devices. Radio frequency glow discharge processes however suffer from relatively slow deposition rates and low utilization of the reaction gas feed stock which are important considerations from the standpoint of making photovoltaic devices from these materials on a commerical basis. In addition, these processes result in high electron temperature plasmas which produce, during deposition, high densities of ions. The production of these ions results in ion bombardment of the materials as they are being deposited which can result in material damage.
A new and improved process for making amorphous semiconductor alloys and devices has recently been discovered which avoids the foregoing mentioned problems. This process is disclosed in copending application Ser. No. 423,424, filed Sept. 24, 1982 for Method Of Making Amorphous Semiconductor Alloys And Devices Using Microwave Energy. This process utilizes microwave energy to decompose the reaction gases to cause the deposition of improved amorphous semiconductor materials. This process provides substantially increased deposition rates and reaction gas feed stock utilization. The process further results in depositions from plasmas with lower electron temperatures and substantially reduced ion densities and hence, substnatially reduced ion bombardment and damage of the deposited materials. Microwave glow discharge processes can also be utilized to make layered structures as also disclosed in application Ser. No. 435,068, filed Oct. 18, 1982, for Method And Apparatus For Making Layered Amorphous Semiconductor Alloys Using Microwave Energy, now abandoned in favor of its divisional application U.S. Ser. No. 565,033, filed 12/23/83;
While these processes have proven successful in the laboratory, they cannot be easily applied to the high volume mass production of photovoltaic devices. The systems disclosed in the aforementioned referenced applications are not suitable for depositing amorphous semiconductor materials onto large area substrates. Furthermore, the disclosed systems are not adapted for the producton of such devices on a substrate which is continuously moved through a plurality of deposition chambers. It is to the solution of these problems and the continuous or batch production of amorphous semiconductor large area photovoltaic devices that the present invention is most particularly directed.